The biogeochemistry of nitrogen in the Atlantic Ocean

Nitrogen plays a fundamental role in limiting the primary productivity of the world's oceans. The Atlantic Ocean is characterised by N-starved oligotrophic gyres to the north and south of the equator, which arise from wind induced Ekman flux of surface waters over the subtropics, subsequent downwelling and the production of a depressed, thicker thermocline. Discrepancies exist between the traditional supplies of N to and the export production from the surface waters of the oligotrophic gyres. This thesis focuses on the importance of N2 fixation and the potential role that organic nitrogen has on fuelling export production in the Atlantic Ocean by examining the biogeochemistry of organic nitrogen. The study formed part of the Atlantic Meridional Transect (AMT) programme and involved three northbound cruises between the Falkland Islands or Cape Town and the U.K., where organic nitrogen was sampled from the euphotic zone of the various biogeochemical regimes of the Atlantic. The spatial extent or' N2 fixation was assessed through the use of stable nitrogen isotope analysis of suspended pa.rticulate organic nitrogen (PONsusp). Consistently depleted signals (with respect to 15N) extend over the centre of the northern subtropical gyre; this partly coincides with a region where the tracer N* increases westward following the gyre circulation. The non-conservative behaviour of N* implies that N2fixation is responsible for the depleted 815N PONsusp• A mixing model suggests that N2 fixation over parts of the northern gyre provides up to 74% of the N utilised by phytoplankton. However, since the PONsusp represents only a small fraction of total N, N2 fixation probably plays a minor role in supplying new N to the photic zone of the northern subtropical gyre. The dynamics of organic nitrogen were investigated through the identification and quantification of dissolved free and hydrolysable, and particulate amino acids. Concentrations along AMT varied with biogeochemical regime; amino acids were depleted in the centres of both the northern and southern oligotrophic gyres and elevated in the higher productivity regions around the equator and temperate zones. The application of the Dauwe and Middleburg [1998] degradation index demonstrated that the 'age' of organic matter also varied, in that more degraded material was found in the gyres compared with the equator and temperate regions. These findings suggest that organic nitrogen can provide a source of N to primary producers. Examination of the enantiomeric ratios of chiral amino acids revealed that bacteria make a minor contribution to dissolved organic nitrogen in the open ocean, contradicting previous claims of bacterial contributions of 45-80%. Bacteria make a more substantial contribution of the particulate organic nitrogen. Surprisingly, archaea dominated the particulate pool, which is consistent studies revealing they are the dominant prokaryote in the world's oceans. Supplied by The British Library - 'The world's knowledge'EThOS - Electronic Theses Online ServiceGBUnited Kingdo